1. Field of the Invention
This invention relates to a method for drawing a glass ingot wherein the glass ingot is drawn to a predetermined outer diameter to obtain a glass rod or preform for an optical fiber having the predetermined outer diameter.
2. Description of the Prior Art
For the drawing of a large-sized glass ingot having a diameter exceeding 100 mm, there is known a method wherein the ingot is heated and softened in an electric furnace, under which it is applied with a given tension thereto.
Such a large-sized ingot has, as shown in FIG. 3, a tapered portion or stepped portion 4 which is formed at opposite sides thereof during the course of its fabrication.
FIG. 5 is a conventional apparatus of drawing a glass rod. For the drawing of such a large-sized ingot 25, dummy shafts provided at opposite sides of the ingot are held so that the ingot is set in the drawing apparatus and is drawn into a glass rod 36.
As an ingot increases in size and diameter, it becomes necessary to further increase the diameter of the heater and a heating zone in the heating furnace.
Where such a large-sized heater is used and an large-sized glass ingot having a tapered portion whose diameter changes in vertical directions is drawn, not only a region of the ingot to be softened and deformed, but also the tapered portion with a smaller diameter is heated and starts to be softened and drawn. Accordingly, the resultant rod cannot have a given diameter throughout its length. As is particularly shown in FIG. 6, there arises the problem that the diameter greatly changes particularly at completion of the drawing.
In this case, there is a further problem that the drawn glass rod should be subjected to finishing drawing by use of a glass-working lathe in order to meet standards for a diameter of the drawn rod, coupled with another problem that the greater change in the diameter results in a longer finishing time.